1. Field of the Invention
This invention relates to improvements in phase-locked loops, and more particularly to improvements in phase-locked loop circuitry to achieve high gain within the bandwidth of the loop while employing amplifiers of moderate gain bandwidth capabilities.
2. Description of the Prior Art
Commonly, instead of employing a single free running oscillator to generate signal frequencies required by some microwave radio systems, two oscillators--a voltage controlled oscillator (VCO) and a reference oscillator are employed together with a phase-locked loop circuit that causes the frequency of the VCO to be controlled by the frequency of the reference oscillator. The output signal is supplied by the usually higher powered VCO which, although of usually poorer frequency stability, because of the phase-locked loop, now has the good frequency stability of the reference oscillator. Proper design of the phase-locked loop circuitry can result in a phase-locked VCO whose long and short term output frequency stability is better than either the free runing VCO or reference oscillator alone. The optimum phase-locked frequency phase stability may call for phase-locked loop circuit gain that is difficult to realize using the prior art circuits.
Digital microwave radio systems employing phase modulation have been encumbered with problems involving intermittent errors, such as in rf sources which experience short duration phase errors which exceed the error threshold of the system. (The term "short duration" is used to denote an event, on the order of less than a millisecond, and perhaps as low as a few nanoseconds.) The problem is often aggravated, since the errors occur only intermittently, and since they occur so fast, the error producing faults are extremely difficult to identify and locate to correct, if such identification and location is possible at all.
Microwave systems typically employ a phase-locked loop in which the phase of a reference oscillator and the phase of a voltage controlled oscillator (VCO) are compared by a phase detector, mixer, or the like. The output of the phase detector is usually amplified and filtered before being supplied to a frequency control input of the VCO. The VCO then outputs a frequency which can be used for signal transmission in the transmitter or as a comparison reference in the receiver. The circuit which includes the phase-locked loop is referred to herein as a frequency source, or sometimes just as a source, with respect to either transmitters or receivers, since both are affected by the same constraints and considerations.
In such microwave systems the rapidly occurring phase errors have been found to result mostly from shifts in frequency by the VCO, and may be caused, for instance, by intermittent electrical contacts in the VCO, or by changes in a VCO load, such as a frequency multiplier, or by mechanical bump on the circuit, or by one of a manifold number of other causes.
When the frequency of the VCO does shift, the phase difference between the VCO and the signal of the reference oscillator starts to change as a result of the shift. The phase error or difference signal output by the phase detector is generally amplified by an amplifier and, in some instances, shaped, and then applied to a VCO input to change the VCO frequency output to compensate for the undesired frequency shift. The compensating signal, however, does not act instantaneously or completely, because such phase error correction would require infinite gain and frequency response in the loop. Thus, to achieve such fast response, the bandwidth of the loop (i.e. the frequency at which the loop gain is one) is made as large as possible. It can therefore be seen that the faster the feedback compensation can be provided, the sooner the phase error can be corrected. At the same time, a maximum amount of loop gain is desired at frequencies below the loop bandwidth because it dereases the percentage of the time that the VCO frequency remains uncorrected, and thereby more nearly meets the requirements of the transmitter or receiver.
The significance of the loop bandwidth to the phase-locked source is that the FM noise of the source output tends to follow the FM noise of the reference oscillator, up to the loop bandwidth frequency. Above the loop bandwidth frequency, the FM noise of the source output tends to follow that of the unphase-locked VCO. Since the low frequency FM noise of the reference oscillator is better than that of the VCO and the high frequency FM noise of the VCO is better than that of the reference oscillator, appropriate choice of the loop bandwidth can produce a phase-locked source with lower FM noise than either the reference oscillator or the VCO alone.
The simplest phase-locked loop (first order) employs an error amplifier with a flat frequency response. The fact that the output of the phase detector (output voltage proportional to phase difference) is applied to the VCO (output frequency proportional to voltage input) gives a loop gain frequency response that decreases 6 db for every octave of increased frequency (with infinite gain at zero frequency).
The most commonly employed second order phase-locked loop employs an amplifier filter with a 6 db gain decrease per octave of increasing frequency over all or most of the loop bandwidth. This amplifier/filter gain functions combined with the 6 db per octave gain slope of the phase detector and VCO combination gives an overall loop gain slope of 12 db over all or most of the loop bandwidth. The 6 db amplifier filter gain slope is most commonly obtained by a flat amplifier followed by a passive RC filter, but in some cases an active filter/amplifier is employed.
Conventional phase-locked loop circuits are typically kept to a second-order loop gain function or less because second-order loop gain is well understood in the art and can be achieved with relatively simple circuitry. (A first-order loop gain function is 6 db per octave; a second-order loop gain function is 12 db per octave.) In a second-order loop, as frequency decreases to values within the loop bandwidth, the gain rises at the rate of 12 db per octave to a given corner frequency and then continues to rise at 6 db per octave as the frequency further decreases.
Third order (18 db per octave) loop gain functions are acknowledged as feasible in the art, but are usually avoided because of their uncertainty and perceived complexity, due to the lack of practical teachings and circuit implementations.
Although third order phase-locked loops are relatively uncommon, it is true that the prior art sources did employ third order phase-locked loops. The amplifier/filter in these sources typically contained an amplifier/active filter followed by a passive filter to realize a 12 db per octave loop gain slope, which resulted in an overall 18 db per octave gain slope.
Frequency or phase modulation noise considerations usually determine the optimum phase-locked loop bandwidth at which frequency the loop gain is unity (0 db). The loop gain rises as the modulation frequencies fall below the loop bandwidth frequency. Higher order loops, because of the higher slopes of their gain versus frequency curves, will have higher gain within the loop bandwidth, and as a result more complete error reduction.
A brute force approach to increasing both the bandwidth and loop gain has been attempted by merely modifying the time constants and the gain levels in the loop. Unfortunately, this simple scaling process calls for a very high amplifier gain-bandwidth product; so high, in fact, that such amplifiers are difficult to fabricate and are extremely costly. In addition, the mere uncontrolled gain increase results in loop saturation or instability, and, hence, may render the device with which the loop is associated essentially inoperable. Another approach to the problem is to simply reduce the loop gain to reduce the amplifier gain-bandwidth product requirement, but this is done at the tradeoff of lower loop gains at low frequencies.
When it was desired to increase the loop bandwidth of the sources it was not possible to do so with the prior art circuit topology because of gain-bandwidth limitations of the available operational amplifiers. It was possible to obtain the wider loop bandwidth with a second order phase-locked loop, but this sacrificed the higher loop gain advantages of the third order loop.